The goal of this research is to investigate the molecular logic of the biosynthesis of complestatin from Streptomyces lavendulae. Complestatin is of medicinal interest because it is a member of the vancomycin group of antibiotics and has interesting therapeutic possibilities with anti-complement and anti-HIV infection activities as well as enhancing plasminogen binding to fibrin, stimulating fibrinolysis. Complestatin is proposed to be synthesized via a non-ribosomal peptide synthetase (NRPS) mechanism that involves the coordinated function of 7 enzyme modules with at least 27 different catalytic domains. This proposal focuses on the enzymatic analysis of 3 modules (1, 2, and 6) of the complestatin NPRS. Each of these modules will be analyzed in isolation in an effort to understand the timing and specificity of residue modification, e.g. chlorination, oxidation, epimerization, and N-methylation. The NRPS subunit containing both modules 1 and 2 will be characterized to determine the timing and substrate specificity of peptide bond formation between residues 1 and 2. The long-term objective of this work is to understand the details of the NRPS mechanism in enough detail to begin manipulating the biosynthetic pathway to generate novel and medically relevant compounds.